The present invention relates to a pump-injector device providing both fuel metering and injection in an internal combustion engine.
There is already known from German Patent specification No. 2,719,228 a device for introducing into an engine a pressurized air-fuel mixture.
From British Pat. No. 447,057 is also known a fuel metering and pumping device which requires the provision of a check-valve between the pumping element and the injection orifice, and U.S. Pat. No. 2,635,590 discloses a device comprising a piston or needle displaceable in a bore which communicates with the injection orifices, fuel metering and injection being achieved during opposed displacements of the piston.
Such prior devices do not generally provide for an accurate timing of the beginning or end of the injection.
There is also known, in particular, from French Pat. Nos. 1,435,259 and 2,027,645, and from the paper "Simulation of the Cummins Diesel Injection System" of Andrew Rosselli and Pat Bagdley presented to the Society of Automotive Engineers (No. 710,570), a fuel injector comprising an injector body having an axial bore whose bottom is traversed by at least one fuel injection orifice or nozzle. A duct supplying fuel under pressure opens in the bottom of this bore. A plunger or needle is slidable in this bore between a first position in which this plunger is spaced from the bottom of the bore, and a second position which is reached at the end of the injection and in which the lower end of the plunger obturates the injection orifices or nozzles.
The displacements of the piston are controlled by an assembly of cam, push rods, rocker arm and return spring.
With such an injector, the injected fuel charge is adjusted by metering the fuel quantity admitted into the injector bore through the fuel supply or inlet duct. Depending on the fuel quantity or charge to be injected, the bore is more or less filled with fuel at the moment where the plunger begins its stroke for fuel expelling, being displaced from its first to its second position. To this end, the fuel inlet orifice is fed with fuel under a pressure varying in relation with the position of the gas pedal and the engine running speed. Thus, the fuel quantity admitted into the bore varies in relation with the inlet pressure and with the duration of the fuel metering period which is inversely proportional to the engine running speed, hence the designation "P-T (Pressure Time) system".
The drawbacks of such a system are to be found, on the one hand, in the difficulty of balancing the flow rates from the different injectors in a multi-cylinder engine, taking into account the importance of a proper calibration of the fuel inlet orifice of each injector, and, on the other hand, in the method itself of regulating the fuel injection, i.e. an automatic control based on the fuel supply pressure.